In recent years, there has been development of organic semiconducting (OSC) materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), photodetectors, organic photovoltaic (OPV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example less than 1 micron thick.
The performance of OFET devices is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance. Further requirements for the semiconducting material are a good processability, especially for large-scale production of thin layers and desired patterns, and high stability, film uniformity and integrity of the organic semiconductor layer.
In prior art various materials have been proposed for use as OSCs in OFETs, including small molecules like for example pentacene, and polymers like for example polyhexylthiophene. However, the materials and devices investigated so far do still have several drawbacks, and their properties, especially the processability, charge-carrier mobility, on/off ratio and stability do still leave room for further improvement.
A promising class of conjugated polymers has been based upon the indenofluorene unit. First reported by Müllen and co-workers [S. Setayesh, D. Marsitzky, and K. Müllen, Macromolecules, 2000, 33, 2016], the homopolymer of indenofluorene was a candidate material for blue light emission in electroluminescent applications. Indenofluorene co-polymers have also been suggested for application as an OSC in transistor devices [WO 2007/131582 A1].
However, the performance of the OSC materials disclosed in prior art is still not always satisfying the current requirements on these materials, and leaves room for further improvement.
In particular, there is still a need for OSC materials that show high charge carrier mobility. Moreover, for use in OFETs there is a need for OSC materials that allow improved charge injection into the polymer semiconducting layer from the source-drain electrodes. For use in OPV cells, there is a need for OSC materials having a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies.
One aim of the present invention is to provide new OSC materials for use in electronic devices, which have the desired properties described above, and do in particular show good processability, high charge-carrier mobility, high on/off ratio, high oxidative stability and long lifetime in electronic devices. Another aim is to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by providing materials as described hereinafter. These materials are based on polymers comprising one or more s-indacenodithiophene or s-indacenodiselenophene units or derivatives thereof, as represented by the following formulae
                s-indaceno[1,2-b:5,6-b′]dithiophene/selenophene        
                s-indaceno[1,2-b:7,6-b′]dithiophene/selenophene(wherein X is S or Se and R1-4 denote for example hydrocarbyl groups),        
It has been found that these polymers are suitable for use as OSC materials in electronic devices, especially in OFETs and OPV cells, and as charge transport layer or interlayer material in polymer light emitting diodes (PLEDs), as they have good processibility and at the same time show a high charge carrier mobility and high oxidative stability.
It was also found that the replacement of the terminal phenyl rings in indenofluorene with thiophene rings increases the electron-rich nature of the conjugated core and therefore increases the HOMO energy level of the resulting homopolymer, poly(2,7-s-indaceno[1,2-b:5,6-b′]dithiophene). In poly(2,8-indenofluorene) the HOMO energy level is low-lying (EHOMO=−5.49 eV as measured by cyclic voltammetry). This increase in the HOMO energy lower allows improved charge injection into the polymer semiconducting layer from the source-drain electrodes (typically made from gold, which has a workfunction of ˜5.1 eV) when applied in a field-effect transistor.
In addition, this additional electron-richness can be exploited to prepare novel low bandgap polymers for photovoltaic applications. By combining the electron-rich s-indaceno[1,2-b:5,6-b′]dithiophene unit and an electron-deficient unit (e.g. benzothiadiazole), resulting co-polymers have a low bandgap (Eg<2 eV). Low bandgap semiconducting polymers are advantageous in bulk heterojunction photovoltaic cells as they enable improved light harvesting by the photoactive layer, which can potentially lead to higher cell efficiencies.
In prior art there is a report of s-indaceno[1,2-b:5,6-b′]dithiophene, but only as a small molecule material [K-T. Wong, T-C. Chao, L-C. Chi, Y-Y. Chu, A. Balaiah, S-F. Chiu, Y-H. Liu, and Y. Wang, Org. Lett., 2006, 8, 5033]. There is also a report of a homopolymer of s-indaceno[1,2-b:5,6-b′]dithiophene [X. M. Hong, and D. M. Collard, Polymer Preprints, 2000, 41 (1), 189], however, this is polymerised at the bridging 4,9-positions and not at the terminal 2,7-positions as described hereinafter.
Furthermore, there is a report of 4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-4,9-dione copolymers [C. Zhao, X. Chen, Y. Zhang, and M-K, Ng, J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 2680.] for application in bulk heteojunction photovoltaics. However, the copolymers reported therein comprise electron-rich thiophene-based co-monomers, instead of the electron-deficient co-monomers that should yield a lower bandgap polymer as desired in this invention.
In addition, there is a report of 4,4,9,9-tetraaryl-s-indaceno[1,2-b:5,6-b′]dithiophene homopolymer and copolymers [S-H, Chan, C-P. Chen, T-C. Chao, C. Ting, C-S. Lin, and B-T Ko, Macromolecules, 2008, published online], again for application in bulk heteojunction photovoltaics. However, the copolymers reported also comprise electron-rich thiophene-based co-monomers, instead of electron-deficient comonomers that should yield a lower bandgap polymer. Furthermore, the tetraaryl substituents at the 4- and 9-bridging positions should not be as effective at solublising the polymers as straight chain or branched chain alkyl substituents. In addition, the tetraaryl substituents should result in poorer solid-state packing of the polymer chains. This is because these aryl rings will twist of the plane of the polymer backbone due to steric interactions, and therefore should prevent the polymer chains from packing closely and result in lower charge carrier mobility.